1. Field of the Invention
The invention is concerned with fibers for use as transmission lines in communication systems operating in the visible or near visible spectra. Such fibers have come to be known as "optical fibers." At the present state of the art, optical fibers are generally glassy--usually based on silica. Forms in use or contemplated for use may be multimode with transmitting core regions many wavelengths in diameter or may have smaller dimensioned cores designed to support only the fundamental mode or a limited number of modes. In either event, fibers, which generally include a clad of relatively low refractive index, and are of an overall dimension of the order of 100 micrometers are drawn from a relatively massive starting body which in terms of composition and refractive index is a large-scale replica of the final fiber. The field of the invention concerns preparation of optical fiber with a particular view to the precise manner in which a starting body or "preform" is prepared.
2. Description of the Prior Art
Lightwave communications involving transmission through optical fiber is fast becoming a significant commercial factor. Trial installations in several countries are in regular use carrying such services as voice, data, and video. Systems may span distances of many kilometers or may involve shorter spacings. Broad band glass fiber systems enjoy cost and/or space advantage relative to copper conductor. Other factors, significant in narrow band and also in short haul use include low weight, radiation--resistance, immunity from electrical interference, and electrical isolation of connected terminals.
While design changes continue to evolve, general fiber configuration is likely to remain unchanged. In such general terms, fibers include a "core" region and an enveloping "clad" region which, depending upon fabrication, may sandwich an intervening "barrier" layer.
Multimode communication fiber designed for use over kilometer distances, is, in accordance with one set of standards, of an outer diameter of 125 micrometers (.mu.m) with a core diameter of 50 .mu.m. An intervening barrier layer is of a thickness of a few .mu.m-perhaps 5 .mu.m. As presently manufactured, this fiber is largely silica. Light guiding qualities are due to an increased refractive index within the core with such increase generally due to doping by a dopant of greater polarizability than that of silica. At this time the primarily dopant is germania. Other compositional variants are responsive to other considerations; and so phosphorus oxide, P.sub.2 O.sub.5, is often introduced into core, clad, and barrier as well to decrease viscosity and so serve as a processing aid. Barrier composition, as the term implies, is designedly such as to block introduction of unwanted contaminant into the core. Such compositional considerations, all exemplary, only, are described in 62, No. 9 Proc. IEEE, 1278 (1974).
Fiber dimensions are those of "multimode" fiber. At usual carrier wavelengths, generally from 0.8 to 0.9 .mu.m and also at contemplated longer carrier wavelengths centering about 1.3 or 1.6 .mu.m, the core is sufficiently large to support thousands of modes. These different modes have different group velocities so that packets of information, generally digital, are composed of many modes traveling at different speeds to result in packet spreading or, for digital communications, in pulse spreading. Multimode fibers designed for use over kilometer distances have cores of radially reducing refractive index designed to lessen this "mode dispersion." See, for example, 52 Bell System Technical Journal, 1563 (1973). This profiled multimode fiber is responsible for the relatively large bandwidth capability of present optical fiber. Operating systems are based on bandwidths of 40-50 megabits/sec. or greater over distances of 10 kilometers or more. Such transmission capacity requires precise control of material concentration gradients responsible for desired refractive index profiles.
It is the general expectation that the trend away from multimode fiber and toward single mode fiber will continue. Mode dispersion is limiting in usual high bandwidth multimode systems. Use of single mode structures takes advantage of low insertion loss values attainable in glass fiber and permits longer inter-repeater spacing. Election of single mode is cost-dependent. Cost factors involve demands placed on terminal equipment, on splicing, and on other design considerations required to accommodate relatively small core dimensions. Initial single mode fiber installations are those in which a premium is placed on maximization of repeater spacings. The primary candidate is for underwater communications use. Longer carrier wavelengths, e.g. in the region of 1.3 .mu.m or even 1.55 .mu.m, inherently permit lower insertion loss and are of particular significance in single mode fiber systems which may be loss- rather than dispersion-limited.
The 1.3 .mu.m wavelength is of particular interest because of very small velocity-wavelength dependence which characterizes usual silica-based materials. This low "material dispersion"--the bulk manifestation descriptive of pulse spreading due to variation in group velocity for different wavelengths--is the result of the crossover between normal and anomalous dispersion wavelength regions. The longer wavelength region at 1.55 .mu.m is characterized by a nonzero anomalous material dispersion which to some extent may offset still smaller realizable values of insertion loss. Fiber may, however, be designed to compensate for this anomalous material disperson by "normal" waveguide dispersion so that systems may take advantage of lower insertion loss. Such fibers depend on core dimensions and index values approaching cutoff for second mode. See 15, No. 12, Electronics Letters, 334-335 (1979).
Insertion losses for fibers traversing distances of kilometers or greater are, in terms of a decade ago, incredibly small. Fiber installations operating at .about.0.82 .mu.m may evidence insertion loss of about 3 db/km. Reported losses for experimental fiber are at 0.5 db/km and 0.2 db/km for wavelengths of 1.3 .mu.m and 1.55 .mu.m, respectively. Extrinsic contributions to insertion loss for the fiber are considered largely due to contaminants--primarily OH. Fabrication techniques are at an advanced level and provide for contaminant exclusion to ppm and better. Close dimensional and composition control are realized as well.
A prevalent fabrication approach involves preparation of a "preform" followed by fiber drawing from a heated tip. In accordance with this procedure, the preform is a solid rod prepared by Modified Chemical Vapor Deposition. See U.S. patent application Ser. No. 828,617, filed Aug. 29, 1977, now U.S. Pat. No. 4,217,027. In a common variant of MCVD, precursor gases, including SiCl.sub.4 and GeCl.sub.4, are introduced as a flowing stream into a tube with a traversing reaction hot zone produced by an external heat source. As practiced, heating is by an oxyhydrogen torch which traverses and retraverses the rotating MCVD tube to result in reaction. Particulate material is carried downstream and is deposited on the tube wall. Deposited particles are consolidated within the hot zone as it advances to produce a smooth, transparent glassy layer. The sequence is repeated many times to result in sufficient buildup of layers of appropriate refractive indices (either consistent or varying) to yield solid preform of desired index profile upon collapse. Replica fiber is then produced by drawing.
This MCVD process is in worldwide use and has resulted in the low insertion loss measurements reported in this description. Low insertion is, in part, attributed to the protected environment of the hot zone which is isolated from the usual combustion heating source so that water of combustion is avoided. The layer-by-layer nature of the process permits close profile control so that bandwidths of gigahertz-kilometer are attained in multimode systems. MCVD has been used for fabrication of single mode fiber as well as multimode fiber.
While the MCVD product is already economically viable, further cost reduction is sought. A promising avenue is increasing preform throughput. A number of parameters contribute to preform throughput, and design advances have resulted in shortened collapse time, in more rapid retraversal, etc. The parameter which has received the most attention, however, is that of reaction and deposition rate.
When MCVD was first introduced, it was clearly deposition-rate limited. Reactant flow under operating conditions resulted in large volumes of particulate matter but in relatively small capture. Under most conditions more reaction product was exhausted then deposited. Studies directed to increased deposition at first identified a mechanism and then yielded increased deposition rates. In accordance with the mechanism, "thermophoresis," particles follow a temperature gradient in the direction of the relatively cool support tube wall. See 50 Journal of App. Phys., 5676 (1979). U.S. patent application Ser. No. 080,483, filed Oct. 1, 1979, now U.S. Pat. No. 4,263,032 describes process variables enhancing deposition through thermophoretic means. An embodiment depends on an enhanced thermophoretic drive field produced by water-cooling the tube downstream of the hot zone. See U.S. patent application Ser. No. 143,845, (Mac Chesney et al 26-1-1), filed Apr. 25, 1980, now U.S. Pat. No. 4,302,230.
An approach to increased reaction rate in MCVD processing is described in U.S. patent application Ser. No. 128,094, filed Mar. 7, 1980, now U.S. Pat. No. 4,262,035. In this MCVD species, an r.f. plasma heat source yields a luminous "fire ball" with temperatures of thousands of degrees centigrade. High reaction rates are permitted, and increased deposition efficiency is ascribed to steep temperature gradients. Unlike flame MCVD, conditions have permitted high reaction rates while avoiding visible particulate matter in the exhaust. A process described as using a microwave plasma in an evacuated chamber is in use in Europe for making fiber preforms. Rates are limited in this plasma Chemical Vapor Deposition process by low reactant introduction rate corresponding with evacuation (Kuppers et al, Technical Digest International Conference Integrated Optics, Optical Fiber Communication--Tokyo, Japan, page 319, 1977).
The "high pressure plasma work" of U.S. application Ser. No. 128,094 supra (in examples open tube end corresponds with ambient atmosphere so that flowing streams in such examples are considered to be atmospheric) permits homogeneous gas phase reaction to yield particulate product in common with other MCVD processes. This is to be distinguished from the rate-limiting heterogeneous wall reaction which characterizes the CVD process as previously practiced in the fabrication of optical fiber (true also of the plasma CVD process). Early promise for high pressure plasma processing has provoked further experimental work. Results have been disappointing. Insertion loss at 0.82 .mu.m has not been below 10 dB/km for high throughput (approaching 1 gm/min deposition). Insertion loss has in general been at levels considered satisfactory for contemplated long-haul communication purposes only at deposition rates attainable in flame MCVD (&lt;5 dB/km for 0.82 .mu.m at &lt;0.5 gm/min). It is commonly thought that plasma MCVD is inherently characterized by a relatively lossy product--a product unsatisfactory for spanning contemplated 10 km or greater interrepeater spacings. See Third European Conference on Optical Fiber Transmission, Munich, p. 15 (1977).